1. Field of the Invention
The present invention relates to a double-sided circuit board including conductive wiring patterns formed on front and back surfaces of a substrate.
2. Description of the Prior Art
A double-sided circuit board is designed to receive electronic components such as semiconductor chips, including chip resistors, chip capacitors, and the like, on the front and back surfaces of its substrate. As compared with the case where electronic components are mounted only on the front surface of a substrate, it is thus possible to improve the total amount of the mounted electronic components over a single substrate. Additional layers of thin film wiring pattern layers over the printed wiring pattern on the front surface of the substrate allow establishment of relatively complicated electric connections or circuits between the mounted electronic components.
As is disclosed in Japanese Patent Application Laid-open No. 5-175657, when the layers of conductive wiring patterns are sequentially formed over the front surface of the substrate, it is necessary to cover and protect a printed wiring pattern on the back surface of the substrate. If the printed wiring pattern on the back surface is exposed to an electrolyte in an electroplating bath, for example, in the process of forming the layers of the wiring patterns on the front surface of the substrate, the printed wiring pattern on the back surface inevitably suffers from electric connections or short circuits induced by unnecessary electroplated film or layer.
The aforementioned Japanese Patent Application Laid-open No. 5-175657 discloses a method employing a protection layer of polyimide designed to keep covering over the back surface of the substrate in the process of forming layers of wiring patterns on the front surface of the substrate. The protection layer of polyimide serves to release the method from repeated formation of a protection layer every time a resist film for forming a layer of wiring pattern is removed from the front surface of the substrate. The process can thus be facilitated. However, the disclosed method is required to completely remove the protection layer of polyimide by using an RIE (Reactive Ion Etching) process or else before another wiring pattern is formed over the printed wiring pattern on the back surface of the substrate.
It is accordingly an object of the present invention to provide a method of producing a double-sided circuit board capable of eliminating a process of removing a protection layer so as to contribute to further facilitation of production process and reduction in production cost.
According to the present invention, there is provided a method of producing a double-sided circuit board including wiring patterns over front and back surfaces of a substrate, comprising: sticking an insulator protection film on the back surface of the substrate prior to formation of a thin film wiring pattern layer over the front surface of the substrate; and mounting an electronic component on the back surface of the substrate, keeping the insulator protection film clinging to the back surface of the substrate, after the formation of the thin film wiring pattern layer.
The method is designed to finally provide double-sided circuit board including the insulator protection film which has served to protect the wiring pattern over the back surface of the substrate in the process of forming the thin film wiring pattern layer on the front surface of the substrate. It is not necessary to subject the substrate to an additional process for removing the insulator protection film before mounting of the electronic component on the back surface of the substrate. The method may contribute to further facilitation of production process and further reduction in production cost. In particular, such a method can be employed when any conductive thin film pattern is not necessarily formed over the wiring pattern on the back surface of the substrate. The electronic component may include an input/output pin, a chip resistor, a chip capacitor, and the like, for example.
In general, a plurality of thin film pattern layers may be formed over the front surface of the substrate after the formation of the insulator protection film. The thin film pattern layers on the wiring pattern over the front surface of the substrate serve to establish a complicated electric connections or circuits between the electronic components mounted on the front and back surfaces of the substrate.
In addition, the method may further comprise subjecting the insulator protection film to a dry process so as to bore a penetrating hole through the insulator protection film when the electronic component is to be mounted. The penetrating hole may be utilized as a conductive via or a conductive through hole. The thus formed via or through hole is designed to establish a reliable electric connection between the electronic component on the insulator protection film and the wiring pattern over the back surface of the substrate.
Such a dry process may employ radiation of a laser beam, for example. In general, when a conductive thin film pattern is not necessarily formed on the wiring pattern on the back surface of the substrate, a laser beam may have ability enough to form a pattern or penetrating hole at a required dimensional accuracy for electrically connect the mounted electronic component and the wiring pattern to each other on the back surface of the substrate. It should be noted that such a dry process may employ any other means for forming a penetrating hole or the like.
According to a second aspect of the present invention, there is provided a method of producing a double-sided circuit board including wiring patterns over front and back surfaces of a substrate, comprising: forming an insulator protection film on the back surface of the substrate prior to formation of a thin film wiring pattern layer over the front surface of the substrate; forming at least a thin film wiring pattern layer over the front surface of the substrate, keeping the insulator protection film on the back surface of the substrate; and boring a penetrating hole through the insulator protection film clinging to the back surface of the substrate.
The method is designed to finally provide double-sided circuit board including the insulator protection film which has served to protect the wiring pattern over the back surface of the substrate in the process of forming the thin film wiring pattern layer on the front surface of the substrate in the aforementioned manner. It is not necessary to subject the substrate to an additional process for removing the insulator protection film before mounting of the electronic component on the back surface of the substrate. The method may contribute to further facilitation of production process and further reduction in production cost.